Fiber lasers have gained market share from more conventional laser sources in various industrial applications, where high optical power levels are required to process a work piece. The rising popularity of fiber lasers is due to their high efficiency, high beam quality, and low need of maintenance.
In order for the fiber laser to function reliably as a whole, the fiber optic components thereof, such as pump or signal couplers, must be constructed in a robust manner. When inside of a high-power fiber laser, the fiber optic components may be subject to high optical radiation fluencies. Most of the optical radiation is designed to be guided by the glass structure of the component when operated in the forward direction. However, under some conditions relatively high optical power levels may also leak out of the fiber optic component into free space inside the mechanical package of the component. As an example, radiation reflected back from the work piece may be guided into the fiber optic component in reverse direction compared to the direction the component has been designed to guide radiation. Such free-space radiation within the package of the component will eventually be absorbed by the package, which results in heating of the package.
When pursuing high reliability of the fiber optic component, the package must be designed so that heating of the package does not damage the component. In order to estimate a conceivable heating power level, let us consider a fiber laser operating at a kW output power level. When such a laser is used in processing a metal with high reflectivity, such as copper, a significant portion of the power affecting the work piece may be reflected back into the fiber laser. Part of this back-reflected power is then converted to heat inside a component package. The heating power level in such a situation may therefore be of the order of 100 W.
Thermally induced stresses to the fiber optic component need not necessarily be generated by radiation as described above. During shipment of a laser system temperature may vary by tens of degrees. As a result of that, failure of a component may occur if such temperature variations have not been considered in the design of the package of the component.
Packages made of metals are often used for mounting fiber optic components in them. Two key parameters should be considered when determining the suitability of a given metal as a packaging material. First parameter is the thermal conductivity, which determines how well the material conducts heat away from the point of introduction of the heat load. The second parameter is the coefficient of thermal expansion, which determines how much the metal expands when it is heated. The fiber optic component itself is usually made of fused silica, whose coefficient of thermal expansion is about 5·10−7/K. Metals typically have coefficients of thermal expansion that are at least ten times higher than that of fused silica. Therefore, if the fiber optic component is rigidly mounted to the metal housing, heating thereof will tend to pull the component and cause it to be strained. As the fiber optic components are often very fragile, such strain may break the component.
One may try to alleviate this effect by using metal alloys having a very small coefficient of thermal expansion, such as invar, i.e. FeNi36. However, the thermal conductivity of such an alloy is often significantly smaller than that of pure copper or aluminum, for instance. Thus the benefit of their low coefficient of thermal expansion is generally counteracted by their low thermal conductivity.
One solution is proposed in U.S. Pat. No. 6,942,399, wherein a optical fiber coupler for joining two optical fibers is disclosed. The coupler according to U.S. Pat. No. 6,942,399 comprises a reinforcing member (20), which is housed stably within a cylindrical member (50). The reinforcing member (20) is made of a hard material, such as quartz, a ceramic or an invar, and has a polygonal cross-section and a longitudinal U-shaped groove (22) for receiving the fiber (30, 31). The fiber (30, 31) is fixed at both ends of the groove by means of adhesives (60). The fiber receiving surface (23) of the reinforcing member (20) is subjected to chrome plating, tin plating, or nickel plating.
However, the proposed construction is unable to effectively minimize strain directed to the fiber optic component as the reinforcing member is made of material that, upon receiving stray radiation from the optical fiber, is unable to transfer the heat away from the fiber.